The invention generally relates to a technique and system for measuring temperature in a subterranean well.
It is often desirable to measure the temperature at several locations in a subterranean well. For example, temperature measurements may be used to observe the movement of an artificially-induced or naturally-occurring temperature region (a xe2x80x9ccold spotxe2x80x9d or a xe2x80x9chot spotxe2x80x9d) in a particular region in the well for purposes of determining a fluid velocity, or flow rate, in that region of the well. Temperature measurements from the well may be used for a variety of other purposes.
There are several different types of temperature measurement systems for use in subterranean wells. A distributed temperature sensor (DTS)-based temperature measurement system uses a sensor that can provide data that is spatially distributed over many thousands of individual measurement points inside the well. One such DTS is an optical fiber, an element whose optical properties are sensitive to its temperature.
When used as a sensor, the optical fiber is deployed downhole so that the optical fiber extends into the region where temperature measurements are to be made. As examples of possible deployment mechanisms, the optical fiber may be deployed downhole with the well casing string or deployed downhole in a conduit that may extend through the central passageway of the casing string.
As a more specific example of a DTS-based temperature measurement system, an optical time domain reflectometry (OTDR) technique may be used to detect the spatial distribution of temperature along the length of an optical fiber. More specifically, pursuant to the OTDR technique, temperature measurements may be made by introducing optical energy into the optical fiber by opto-electronics at the surface of the well. The optical energy that is introduced into the optical fiber produces backscattered light. The phrase xe2x80x9cbackscattered lightxe2x80x9d refers to the optical energy that returns at various points along the optical fiber back to the opto-electronics at the surface of the well. More specifically, in accordance with OTDR, a pulse of optical energy typically is introduced to the optical fiber at the well surface, and the resultant backscattered optical energy that returns from the fiber to the surface is observed as a function of time. The time at which the backscattered light propagates from the various points along the fiber to the surface is proportional to the distance along the fiber from which the backscattered light is received.
In a uniform optical fiber, the intensity of the backscattered light as observed from the surface of the well exhibits an exponential decay with time. Therefore, knowing the speed of light in the fiber yields the distances that the light has traveled along the fiber. Variations in the temperature show up as variations from a perfect exponential decay of intensity with distance. Thus, these variations are used to derive the distribution of temperature along the optical fiber.
In the frequency domain, the backscattered light includes the Rayleigh spectrum, the Brillouin spectrum and the Raman spectrum. The Raman spectrum is the most temperature sensitive with the intensity of the spectrum varying with temperature, although all three spectrums of the backscattered light contain temperature information. The Raman spectrum typically is observed to obtain a temperature distribution from the backscattered light.
Another technique that may be used in conjunction with a DTS-based temperature measurement system is an optical frequency domain reflectometry (OFDR) technique. As is known in the art, OFDR is not time domain based like the OTDR technique. Rather, OFDR is based on frequency.
Another type of temperature measurement system embeds gratings, called fiber Bragg gratings (FBGs), in an optical fiber for purposes of sensing downhole temperatures. An FBG-based temperature measurement system is described in, for example, U.S. Pat. No. 5,380,995. These Bragg gratings may also measure strain.
Fiber Bragg gratings are manufactured by a variety of methods inside the core of standard telecommunications grade single mode fiber. Referring to FIG. 1, a standard single mode fiber 1 includes an eight micron diameter core glass material 2 that is surrounded by a 125 micron cladding glass material 3, of a different index of refraction, that gives the fiber 1 its waveguide properties. A fiber Bragg grating 4 is photowritten onto the core material 2 by ultraviolet laser radiation and represents a 4-6 mm long periodic modulation in the core""s index of refraction by approximately 0.01%.
This perturbation in the core yields a Bragg wavelength, xcexB, given by Bragg""s law:
xcexB=2 nc xcex9,xe2x80x83xe2x80x83(1) 
where nc is the effective core index of refraction and xe2x80x9cxcex9xe2x80x9d is the period of the index modulation. The reflection peak, xe2x80x9cxcexB,xe2x80x9d is linear with strain (called xe2x80x9cxcex5xe2x80x9d) and temperature (called xe2x80x9cTxe2x80x9d) and is described by the following equation:                                                         Δ              ⁢                              xe2x80x83                            ⁢              λ              ⁢                              xe2x80x83                            ⁢              B                                      λ              ⁢                              xe2x80x83                            ⁢              B                                =                                                    (                                  1                  +                  ξ                                )                            ⁢              Δ              ⁢                              xe2x80x83                            ⁢              T                        +                                          (                                  1                  -                                      ρ                    e                                                  )                            ⁢              ϵ                                      ,                            Eq        .                  xe2x80x83                ⁢                  (          2          )                    
where "xgr" and xcfx81e are the thermal optics (dn/dT) coefficient and the photo elastic coefficient, respectively. Effectively a strain and temperature gauge inside an optical fiber, the FBG has demonstrated linear response down to nanostrain levels and up to 500 degrees Centigrade. During the manufacturing process, the period of modulation in the index of refraction can be adjusted to produce multiple FBGs on a single fiber each with a unique center Bragg wavelength, xcexB. The FBG-based system is therefore suitable for a multi-sensing system with a single optical fiber line because wavelength domain multiplexing (WDM) and time domain multiplexing (TDM) can be applied.
FIG. 2 depicts a conventional system 5 that uses FBGs. The system 5 includes an incoherent broadband light source 6 (with 50 nm bandwidth) that is inserted into a fiber optic cable 7 that has several FBG""s 8 written onto it at different spatial locations. The system 5 also includes a detection subsystem 9. Each FBG 8 reflects a narrow band fraction (typically, 0.2 nanometers) of the broadband source light with a unique wavelength (xcex1,xcex2, . . . ) encoded tag. The FBG""s may be a few millimeters or kilometers apart, but they will maintain the same wavelength separation. As each Bragg grating 8 is subjected to strain or temperature variations, the center Bragg wavelength will move to shorter or longer wavelengths, independently of the others, and it is this wavelength change that is measured by the demodulation detection system shown.
FIG. 3 shows a spectral waveform 12 of a single Bragg grating as a function of wavelength. Also depicted in FIG. 3 is a waveform 10 of the source light emitting diode (LED) 12. Typically, the demodulation system is attached via a fiber optic beamsplitter where a fraction of the returned light from the FBG is diverted from its return to the light source and into the demodulation system. FIG. 4 depicts the reflection response of two FBG""s (depicted by spectral waveforms 13 and 14) when illuminated with a broadband light emitting diode (having a spectral waveform 15) in the near infrared band centered at 1300 nanometers. It will be understood by those skilled in the art that all the Bragg gratings in FIGS. 3 and 4 may have the same wavelength and the light source and demodulation system may operate in a time division multiplexing mode thereby identifying each FBG by light travel time in the fiber, rather than wavelength. The thermal sensitivity of each grating is still governed by equation 2. It is further understood that the thermal response of the grating may be enhanced mechanically by utilizing the strain response of a Bragg grating by coupling the FBG to a material with a large Coefficient of Thermal Expansion. A method for this is described in U.S. Pat. No. 6,246,048.
Other kinds of sensors to measure physical and chemical sensors include interferometric sensors and attenuation based sensors.
In an embodiment of the invention, a technique that is usable in a subterranean well includes deploying a first sensor in a remote location to measure a distribution of a characteristic along a segment at the location. The technique includes deploying a second sensor downhole to measure the characteristic at discrete points within the segment. The second sensor is separate from the first sensor.
Advantages and other features of the invention will become apparent from the following description, drawings and claims.